Rapid viscous flow of crustal rocks controls dyke emplacement in the ductile crust

Magmatic dykes are the main pathways for magma through Earth's crust. They are often assumed to be magma-filled fractures that propagate as thin, tapered sheet intrusions through mode I tensile opening at the fracture tip and elastic bending of the host rock along the dyke walls. Here, we present field evidence from northern Sweden, showing that deep-crustal dyke emplacement was, despite high strain rates, associated with significant ductile deformation of the host rocks. On average, 25% of the dyke thickness was accommodated by ductile flow of the immediate host rock. Modelling magma cooling times suggests average ductile strain rates of 10^-3 s^-1 to 10^-6 s^-1, i.e. 6 to 10 orders of magnitude faster than typical tectonic ductile strain rates in the middle crust. These results have major implications for understanding ductile crustal strength and the interpretation of geophysical signals used to mitigate geohazards in volcanically active areas.